Curing method for polyether

ABSTRACT

This invention relates to a novel curing method of oligomers, using metal triflates, and particularly to the curing of hydroxyl terminated elastomers to achieve crosslinked polymers. The method finds particular use as an alternative cure methodology to replace isocyanate curing. There is further provided a cured and crosslinked polymer binder, which is particularly suitable and compatible for use with energetic materials.

This invention relates to the field of curing oligomers, particularlythe curing of hydroxyl terminated elastomers to achieve crosslinkedpolymers. The method finds particular use as an alternative curemethodology to replace isocyanate curing. There is further provided acured and crosslinked polymer binder, which is particularly suitable andcompatible for use with energetic materials.

Isocyanates, or di-isocyanates, are basic constituents in the productionof polyurethane. Methods of use vary but the essential process is thatthey combine with hydroxyl terminated polymeric resins causing areaction which creates durable polyurethane foams, elastomers, paints oradhesives. Whilst isocyanates are extremely useful, they are known to beirritants and highly potent sensitising agents which affect the skin andthe upper respiratory tract.

Elastomers are commonly cured with approximately 1 to 10% isocyanates tofor n polyurethanes. There are numerous uses for the resulting rubberssuch as o-rings, rollers, belts, damping elements, sleeves, valve seatsetc. Such materials are widely used in a huge number of industries suchas agriculture, transport, aerospace, building, furniture industriesetc. Numerous types of hydroxy terminated elastomers are cured withisocyanates—these include various types of polyethers, polyesters,polyalkanes and polyalkenes. As well as the above uses, elastomers areused extensively as binders for composite rocket motors.

The advantage of using isocyanates as a curing agent is their ability toreadily react with many functional groups under mild conditions.However, isocyanates have many drawbacks such as toxicity and theirhazardous production, the Bhopal disaster being the most significantexample. Their excellent reactivity makes them very useful; howevertheir use is becoming more limited due to tighter safety regulations.However, lack of alternative processes and the industrial importance ofthe resulting polyurethanes have so far outweighed the risks ofisocyanate manufacture and processing.

According to a first aspect of the invention there is provided a methodof forming a crosslinked polyether polymer comprising the steps offorming an admixture of at least one hydroxy terminated oligomer, atleast one epoxy terminated oligomer and at least one metaltrifluoromethanesulfonate salt catalyst, and curing the resultantadmixture at an elevated temperature.

It is desirable that at least one of the hydroxy or epoxy oligomerspresent is comprised of at least 5 to 10% w/w of an oligomer which hasin the range of 3 to 5 hydroxy or epoxy groups respectively, to promotecross linking.

In a further embodiment the admixture comprises a further epoxyterminated oligomer, which contains an average of 2.5 to 4 epoxy groupsper oligomer chain, and is present in the range of from 5-10% w/w of theadmixture.

The hydroxy and/or epoxy terminated oligomers may comprise a non-integernumber of hydroxy groups or epoxy groups per oligomer chain, due to themethods of synthesising oligomers. Clearly, any side reactions whichoccur during the synthesis of these oligomers may result in theformation of branched chains. These branched chains may also possess ahydroxy or epoxy unit at the end of said branch, thereby causing theoligomer (as a mixture) to possess a non-integer number of hydroxy orepoxy groups.

Crosslinked polymers that are particularly suitable for binders forenergetic materials desirably comprise polyhydroxy terminated oligomers,wherein the oligomer is of formula (i)

wherein A is an oligomer containing monomer repeat units, m is theaverage number of monomer repeat units in the range of from 1 to 10000,and x is in the range of from 2 to 20; andthe polyepoxy terminated oligomer is of formula (ii)

wherein B is an oligomer containing monomer repeat units, n is theaverage number of monomer repeat units in the range of from 1 to 10000,and y is in the range of from 2 to 20.

The crosslinked polymer may comprise two or more independently selectedformula (i) components and/or two or more independently selected formula(ii) components. The at least one hydroxy terminated oligomer ispreferably selected from oligomer units which are terminated with atleast two hydroxyl groups, preferably the number of hydroxyl groupspresent per oligomer, i.e. x is preferably in the range of from 2 to 10,more preferably in the range of from 2 to 4.

In an alternative arrangement the hydroxy terminated oligomer is offormula (i),

wherein A and m are as hereinbefore defined and x is in the range offrom 1 to 3; and the epoxy terminated oligomer is of formula (ii)

wherein B and n are as hereinbefore defined and y is in the range offrom 1 to 3, and the admixture comprises a further epoxy terminatedoligomer of formula (iii), present in the range of from 5-10% w/w,

wherein D is an oligomer containing monomer repeat units, p is theaverage number of monomer repeat units in the range of from 1 to 10000and z is in the range of from 2.5 to 3.

The molar ratio of functional end groups in formula (i):formula (ii) maybe varied preferably the ratio is of from 0.65:1, more preferably0.85:1, yet more preferably the hydroxyl and epoxy groups are present inthe ratio of 1:1 i.e. substantially equimolar ratio. When the ratio isequimolar, preferably 5-10% w/w of at least one of the hydroxyterminated oligomer or the epoxy terminated oligomer contains an averageof 2.5 to 3 hydroxyl groups per oligomer chain. Alternatively it may bedesirable to provide the equimolar admixture with 5-10% w/w of thefurther epoxy terminated oligomer. The use of 5-10% w/w of oligomer withgreater than 2 functional groups (i.e. epoxy or hydroxyl) will help topromote crosslinking in the final crosslinked polymer.

Preferably the hydroxy terminated oligomer has substantially twoterminal hydroxy groups and the epoxy terminated oligomer hassubstantially two terminal epoxy groups, such that the ratio of hydroxylto epoxy is substantially 1:1, preferably the admixture comprises atleast one further epoxy terminated oligomer, which contains an averageof 2.5 to 3 epoxy groups per oligomer chain, and is present in the rangeof from 5 to 10% w/w.

Preferably, the polyhydroxy terminated oligomer is a di-hydroxyterminated oligomer of formula (ia)

wherein A and n are as hereinbefore defined, the polyepoxy terminatedoligomer is di-epoxy oligomer of formula (iia)

wherein B and m are as hereinbefore defined and optionally the admixturecomprises a further epoxy terminated oligomer of formula (iii), presentin the range of from 5-10% w/w,

wherein D, p and z are as hereinbefore defined.

The oligomer may comprise a series of repeating monomeric units of A, Bor D, such as, for example hydrocarbyl, esters, carbonates, ethers,amides, aromatics, heterocyclic or copolymers comprising mixturesthereof. The hydrocarbyl may be optionally interposed with heteroatoms,esters, carbonates, ethers, amides, aromatics or heterocyclic groups,the hydrocarbyl may also optionally substituted with functional groups,such as for example halo, nitro, haloalkyl, short chain C₁-C₆ alkyl. Thehydrocarbyl may be a combination of straight or branched chain alkyl,alkenyl or alkynyl groups. Preferably the oligomer of formula (i) is anelastomer, i.e. one which possesses similar elastic properties to thatof natural rubber.

The monomeric units A, B or D (when present) may all be the same,preferably the oligomer backbone of the hydroxy oligomer A is selectedfrom a different oligomer to the epoxy oligomer B or D. The oligomers ofA, B and D may themselves be in the form of homo-oligomers, i.e. allmonomeric units within an oligomer may be the same or co-oligomers suchthat the monomeric units within the oligomer are selected from differentmonomeric units. The co-oligomers may be arranged such as, for example,as block, random, or statistical co-oligomers.

The number of repeat units of m, n and p are in the range of from 1 to10,000, more preferably m, n and p are in the range of from 5 to 100units, so as to provide oligomers with an average molecular weight lessthan 10,000. More preferably m, n and p are in the range of from 20 to50 units to provide oligomers with average molecular weights of lessthan 5000.

Preferably the epoxy terminated oligomer and the further epoxyterminated oligomer, when present, possess fewer units than the hydroxyterminated oligomer. Preferably the epoxy terminated oligomers, haveless than 100 units, preferably less than 50 units, more preferably 1 to10 units. Preferably the hydroxyl terminated oligomers have less than100 units, more preferably 1 to 50 units.

It will be clear that oligomers, due to their method of synthesis, aretypically represented as having an average number of units, such as forexample a 20 unit oligomer may actually have a spread of oligomers from10 to 30 units.

A particularly preferred system is the use of hydroxy terminatedpolybutadiene (HTPB) with an average molecular weight range of 1200 to2500 (indicative of a spread of different length monomer units,typically between 22 to 50 units), in combination with an epoxyterminated oligomer with 1 to 5 units.

Examples of hydroxy terminated oligomers are

HTPB is particularly useful oligomer of formula (i), which comprises aplurality of repeating butadiene monomer groups. HTPB is currently usedin crosslinked polymers designed to be used for energetic materialbinders, as it possesses a low glass transition temperature, lowmoisture capacity, good processing properties and good mechanicalproperties for cured elastomers.

Examples of epoxy oligomers are:

It would be clear that the hydroxyl groups of oligomer A could bereadily replaced with epoxy functionality, and similarly the epoxygroups oligomer B and D could be replaced with hydroxyl groups.

The component repeat units which make up the oligomer A, B and D willprovide the final cross linked polymer with different physical andchemical properties, such as, for example the degree of—hydrophobicityor hydrophilicity. The rigidity and glass transition temperatures of thefinal cured crosslinked polymer will be determined by both theappropriate selection of the oligomer backbone A, oligomer B andoligomer D, when present, and also the degree of cross linking thatoccurs.

Preferably the metal of the at least one metal triflate is a rare earthmetal, a group III metal or gallium, which is known to possess similarchemical properties to rare earth metals. The group III metals arepreferably selected from scandium or yttrium. Preferably the metal is alanthanide or actinide, more preferably a lanthanide, yet morepreferably lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, erbium, thulium orytterbium.

The metal is preferably scandium or yttrium, as they provide increasedrates of reaction and are more cost effective.

Yet more preferably the catalyst comprises scandium (III)trifluoromethanesulfonate, yttrium (III) trifluoromethanesulfonate,samarium (III) trifluoromethanesulfonate, europium (III)trifluoromethanesulfonate, erbium (III) trifluoromethanesulfonate,ytterbium (III) trifluoromethanesulfonate or cerium (IV)trifluoromethanesulfonate. The ligand trifluoromethanesulfonate iscommonly referred to an abbreviated form of triflate, TFMS or (OTf).

A rare earth metal catalyst preferably does not have any other ligandspresent. It may not be desirable to incorporate further ligands onto therare earth metal, as this may cause steric hindrance, as the use ofbulky coordinated ligands may affect the ability of the rare earth metalsalt to successfully coordinate with the oxygen atoms on the epoxyoligomer.

In order to delay the onset of curing, it may be desirable to form apre-admixture comprising any two of the at least one polyhydroxyterminated oligomer, at least one polyepoxy terminated oligomer and atleast one metal trifluoromethanesulfonate salt catalyst, and then addthe final component at a selected time to cause the curing of theadmixture. This would allow ready storage of the components to form atwo component mix, which could be cured by forming the admixture andcausing curing to occur.

There is further provided a kit for use in the above method comprisingthe following components either all provided separately in the kit, orwith any two provided in combination and the third provided separately:—

at least one hydroxy terminated oligomer;at least one epoxy terminated oligomer;at least one metal trifluoromethanesulfonate salt catalyst; and,wherein the kit further comprises optional instructions for use.Preferably, the respective components are provided in suitable ratios toproduce a useful crosslinked polyether polymer. The catalytic componentmay be combined with either oligomer.

Preferably the admixture of the hydroxy terminated oligomer and theepoxy terminated oligomer is formed in the substantial absence ofsolvent. The catalysts are solids in the form of salts, therefore inorder to aid the incorporation of the catalyst into the admixture, itmay be desirable to dissolve the catalyst in a minimum quantity of avolatile solvent prior to adding to the admixture. Preferably thesolvent is removed prior to curing the admixture, such that curing takesplace in the substantial absence or complete absence of solvent.Typically the solvent that is used to dissolve the catalyst may be aketone, ether, alcohol or an aprotic solvent.

It is desirable to carry out the curing reaction in the substantialabsence of solvent as certain oxygen containing aprotic solvents mayreduce the activity of the catalyst, such as for example tetrahydrofuranor dioxane. In these cases it is possible that the catalyst maycoordinate with the solvent in preference to the epoxide functionality.Further advantages to performing the curing stage in the substantialabsence of solvent are reducing the cost of handling, storing anddisposing of the solvent. A further benefit is reducing the likelihoodof inclusion of trapped solvent in the final cured crosslinked polymer.

The metal triflates are only required in catalytic amounts, whichsignificantly reduces the cost of processing and curing the admixture.Preferably the catalyst is present in an amount of from 0.01% to 2% bymass of the reaction mixture, more preferably the catalyst is present inthe range of from 0.05 to 1%, more preferably in the range of from 0.2to 1%. The catalyst may be left in the admixture, it may be recovered,or may be incorporated as part of a solid support, which may later beremoved.

There is further provided a composition comprising a crosslinkedpolyether polymer, which polymer includes residual catalytic amounts ofat least one metal trifluoromethanesulfonate salt catalyst.

As the reaction is preferably carried out in the substantial absence ofsolvent the curing reaction does not need to be subjected to reducedpressures in order to remove the solvent, and hence may be readilycarried out at atmospheric pressures. Preferably the curing step iscarried out at an elevated temperature, i.e. above room temperature, andpreferably the curing step is carried in the temperature range of 30 to80° C., more preferably 40 to 60° C. It may also be desirable to formthe admixture at an elevated temperature in order to reduce theviscosity of the oligomers. The reaction according to the inventionprovides a desirable alternative to isocyanate curing of polyhydroxyterminated oligomers, without the use of toxic or corrosive reagents.

A further problem with the use of isocyanates is that they are reactivewith water and so will readily decompose to release carbon dioxide ifthey come into contact with moisture i.e. such as, for example frominsufficiently dried starting materials. Advantageously the catalyticactivity of metal triflate catalysts is not affected by the presence ofmoisture. The formation of the admixture and even the curing step of thereaction may, if desired, be carried out in the presence of water or aprotic solvent. However, the subsequent removal of said solvent, priorto or during curing, would still need to be addressed.

According to a further aspect of the invention, there is provided acured polymer obtainable by the method according to the invention.

In a further embodiment the admixture further comprises at least onefiller material. It may be desirable to incorporate fillers i.e.additives into the admixture to alter the properties of the crosslinkedcured polymer. Conveniently the filler is added to the admixture andmixed to form a uniform dispersion of the filler and admixture, prior tocuring. The fillers may be selected from those routinely used in polymerchemistry, such as, for example, plasticisers, stabilisers,antioxidants, inert fillers, photostabilisers, pigments, etc.

In a preferred embodiment the filler is an energetic material, i.e. anexplosive, such as, for example, a high explosive or propellantmaterial.

According to a further aspect of the invention there is provided amethod of forming a polymer bonded explosive, comprising the steps offorming an admixture as defined hereinbefore, uniformly dispersing anenergetic material in said admixture and curing the resultant mixture atan elevated temperature; optionally the mixture may also contain aplasticiser.

Plasticiser are commonly used to lower the glass transition temperatureand improve low temperature mechanical properties. Plasticisers areparticularly important for use in creating binders for use withenergetic materials, as they may improve processability, decreaseshazard properties, and can increase the performance of energeticcomposites.

The method according to the invention finds particular advantage in itsuse in curing oligomer units to form a crosslinked polymer which issuitable for use with energetic materials. It is a well documented thatisocyanates are incompatible with certain energetic fillers, such as,for example, ammonium dinitramide. Ammonium dinitramide is a well knownrocket motor propellant oxidiser, which is being increasinglyincorporated into rocket motors due to its good combustion properties.

Accordingly there is provided an explosive formulation comprising atleast one energetic material uniformly dispersed in at least one curedpolymer prepared by a method according to the invention. The curedadmixture may be present in the range of from 1 to 40 w/w of theexplosive formulation.

There are other reagents which can activate the reaction of an alcoholand an epoxy to afford a polyether. Typical reagents may be aqueousstrong acids or bases, tributyl tin complexes or boron trifluoride. Inthe case of the latter two examples, the reagents are also toxic,moisture sensitive and as such their use would not be well suited toindustrial application. Moreover, the above reagents would not beparticularly well suited to be used in the processing of energeticmaterials, due to the possibility of adverse reactions.

As a yet further advantage, lanthanide triflates, at the concentrationsused in the curing oligomers of the invention, have been shown to becompatible with ammonium dinitramide and as such there are no adversereactions, even after prolonged and continued exposure to the lanthanidetriflates. Therefore any trace amounts of rare earth metal catalyst thatmay be present in the final cured polymer will not cause a problem tothe safety and stability of the final explosive formulation. Thereforethe method according to the invention is particularly suited to providecrosslinked polymers as binders that are compatible and safe to use withenergetic materials (explosive materials) that are susceptible toreaction with isocyanate groups.

For explosive formulations, there is a requirement that oligomer repeatunits A, B and D, and the final crosslinked polymer are inert, stable,and compatible with energetic fillers.

The final crosslinked polymer should have a low glass transitiontemperature, high moisture resistance, be miscible with plasticisers andimmiscible with solid energetic fillers. The resultant polymericcomposites should possess hazard properties that are mitigated by thepolymeric binder and mechanical properties such as tensile strength andstrain capability that are suitable for use in solid propellant rocketmotors, and plastic bonded explosive applications. Composites based onpolybutadiene, polycaprolactone and other polyesters are commonly usedfor energetic polymeric compositions.

EXPERIMENTAL

A general reaction scheme is shown below in reaction scheme 1.

Example 1 General Polymer Preparation

The polyhydroxy terminated oligomers, as shown in Table 1, weretypically dried in vacuo at 50° C. overnight whilst stirring. It wasobserved that if the catalyst was added to the polyhydroxy terminatedoligomer as a solid and stirred that the dissolution times were slow andcrystalline matter remained. Hence, in all subsequent preparations, thecatalyst was dissolved in a minimum aliquot of acetone and stirred in tothe oligomer to form a pre-admixture. The acetone was then removed byapplication of a vacuum.

The epoxy terminated oligomer component was added to theoligomer/catalyst pre-admixture to form the admixture, which was stirredin vacuo at 50° C. for 10 minutes until the epoxy terminated oligomerhad been thoroughly mixed. The samples were poured into PTFE moulds andcured for at least a week at 60° C.

The crosslinking process and epoxy consumption was followed by dynamicstress rheology (DSR) and Fourier transform infrared spectroscopyrespectively. The mechanical properties of the crosslinked curedmaterial were studied using dynamic mechanical analysis (DMA)spectroscopy.

Hydroxy Terminated Oligomer Variation Analysis of Gel Time

Gel time is a function of both the crosslinker molecule and the catalystconcentration, and is defined at the time when the elastic moduluscrosses the damping modulus. Dynamic Stress Rheology (DSR) measurementswere used to follow gel times of curing HTPB at 60° C. The gel time ofthe system can be tailored to meet end use requirements.

The progress of the gel reaction was monitored, for several differentcatalyst and oligomer A and B compounds using proton nuclear magneticresonance spectroscopy (HNMR). It was observed that the proton on thesecondary alcohol, generated from the ring opening of an epoxide,remains present on the spectra during the course of curing. Thisindicates that the secondary alcohol does not itself undergo reactionwith a further epoxide ring at the same rate as that observed forprimary hydroxyl groups.

TABLE 1 Gel time of hydroxy terminated oligomers cured with epoxyEpikote 828 and europium TFMS catalyst. Shear 1^(st) order Modulus rateat 60° C./ Sample Gel time constant MPa Desmophen 1800 >24 0.00048 0.33polycaprolactone >24 0.0011 0.18 polyhexamethylene phthalate 4.0833330.0067 0.85 polyhexamethylene carbonate diol >24 0.00084 0.18polyethylene-co-1,2-butylene Not 0.0067 1.17 measured HTPB 1.41 0.0740.53 Polypropylene glycol polyethylene 3.5 0.0067 0.17 glycol blockco-polymer polynimmo pp570 >24 0.00735 0.065 PolyGLYN Batch 3.24 200.001 0.85

Table 1 summarises the typical 1^(st) order curing rates (as measured byinfrared spectroscopy) and the modulus of the final materials asmeasured using Dynamic Mechanical Analysis (DMA). The molar ratio of thepolyhydroxy oligomer hydroxy end groups to epoxy end groups was 1:1,catalysed by 0.6% by mass Eu(TFMS)₃ and the cure was carried out at 60°C.

The crosslinking reaction was found to vary on the polymer type. Thecuring process for oligomer units that comprise polyether monomericunits tended to be less efficient than that measured for the monomericunits containing polyesters, HTPB and polyalkanes. This may be due tothe oxyphilic behaviour of the lanthanide catalysts. Coordination of theLnTFMS on to the ether linkage, of the polyether, may decrease theeffectiveness of the catalyst and thereby decrease the rate of reaction.

Comparison of Isocyanate and Epoxy Cross Linked Polymers

Table 2, below, indicates the tensile testing data of the admixture ofHTPB and epoxy Epikote 828 cured by the method according to theinvention. For comparison a polymerisation which uses isocyanate isprovided. As a further comparison plasticized and un-plasticizedexamples were also prepared, the plasticiser was bis(2-ethylhexyl)sebacate.

TABLE 2 Comparison of tensile data of isocyanate cured polybutadienewith epoxy cured polybutadiene. Tensile Maximum Modulus/ Stress/ BreakMaterial MPa MPa Strain/% HTPB + isophorone di-isocyanate 1.0 0.4 55HTPB + isophorone di-isocyanate + 0.2 0.1 66 25% dioctyl sebacateplasticiser HTPB + Epikote 828 + ErTFMS 1.4 0.3 26 HTPB + Epikote 828 +ErTFMS + 1.1 0.2 22 25% dioctyl sebacate plasticiser

The physical properties were measured using Instron tensile testing. Itcan be seen from the results in Table 2 above that the highest modulusand maximum stress are for the unplasticised materials. The strain atbreak is lower for the epoxy cured materials because they are morecrosslinked than the isocyanate materials.

The mechanical properties of cured elastomers over a range oftemperatures were measured using dynamic mechanical analysis (DMA). TheDMA traces of isophorone di-isocyanate cured HTPB (cured at 60° C. forone week) were found to be similar to the epoxy cured HTPB. Thereforemethods of synthesis according to the invention provide polymers withsimilar mechanical properties to that of isocyanate cured polymers.

The lanthanide trifluoromethane sulphonate catalysts have been shown tosignificantly accelerate the epoxy ring opening process compared toun-catalysed reactions. The gel time in the presence of La(TFMS)₃ (5.38hours) was more than double that measured for dysprosium and thuliumTFMS. However, for most of the catalysts, the gel time did not varygreatly; this reflects the similar chemistries exhibited across thelanthanide series. The elements at the latter end of the series,however, do appear to accelerate epoxy ring opening faster than thoselanthanides at the beginning of the series. This might reflect changesin Lewis acidity caused by the lanthanide contraction as the atomicnumber is increased.

Epoxy Variation

The effect of different epoxide oligomer materials on the curing of HTPBin the presence of one particular catalyst EuTFMS was undertaken and theresults are provided in Table 3, below.

TABLE 3 Effect of epoxy crosslinking agent on the curing of HTPB 1^(st)Hydroxy Gel time/ order rate Shear Modulus Epoxy oligomer oligomer hoursconstant at 60° C./MPa neopentyl glycol HTPB 7.5 0.098 0.08 diglycidylether trimethylolpropane HTPB 5.9 0.0054 0.23 triglycidyl ether Epikote828 HTPB 3.4 0.0087 0.69

The gel time and rate of reaction varies depending on the epoxide used.There is no linear relationship between the rate of consumption of theepoxy ring, (i.e. ring opening) versus the gel time rate. This may bedue to complex interactions of hydroxy oligomer functionality, epoxyfunctionality and changes in diffusivity due to network formation i.e.crosslinking, during the cure. The epoxy and hydroxyl oligomers werecurable in a desirable time period.

HTPB Cure Using Variety of Lanthanide Triflates

HTPB (containing 1% by mass of calco2246 (antioxidant)) was dried invacuo at 50° C. overnight. Epikote 828 (available from Aldrich) wasadded such that there was 1:1 mol equivalence of epoxy to hydroxylgroups. A selection of lanthanide metal triflate catalysts were added inan amount of 0.1 mmol equivalent of catalyst per g of HTPB/Epikote 828.The catalyst was dissolved in a minimum quantity of solvent prior toadding to the mixture, which was subsequently removed under vacuo. Theadmixtures were cured at 60° C. in a fan oven for 7 days. The mechanicalproperties as measured by DMA are compared to a rubber made fromisophorone di-isocyanate (IPDI) and HTPB.

TABLE 4 Optimisation of HTPB cure. Gel 1^(st) Order Shear Shear Time/rate Tg/ Modulus at Modulus at Crosslinker Catalyst Mass % hoursconstant ° C. 60° C./MPa 25° C./MPa Epikote Ce(TFMS)₃ 0.64 3.17 0.0090−74 1.64 1.59 828 Epikote Dy(TFMS)₃ 0.61 2.76 0.010 −77 0.68 0.62 828Epikote Sm(TFMS)₃ 0.59 3.60 0.0087 −77 0.69 0.63 828 Epikote Yb(TFMS)₃0.53 4.19 0.0080 −74 0.71 0.63 828 Epikote Tb(TFMS)₃ 0.60 2.97 −77 0.760.68 828 IPDI — — — — −68 0.41 0.40

The above curing reactions were followed using both infraredspectroscopy (FTIR) and dynamic stress rheology (DSR).

In the case of FTIR measurements, it was found that the resolution ofthe epoxy peak at 1250 cm⁻³ was poor, hence the epoxy content wasfollowed using the epoxy combination band at 4541-4510 cm⁻³ in the nearIR region.

A typical dynamic stress rheology (DSR) plot of curing material at 60°C. revealed that the shear modulus of the formed cross linked polymermaterial increased rapidly within the first three hours. Despite usingdifferent catalysts, the gel times were similar—typically 3-4 hours.

Beyond the gel point, the polymer network does not flow. Hence themeasurements in Table 4 above, suggest that there is too much catalystin the reaction mixture for explosive and propellant formulation. Forthe purpose of energetic binder manufacture, a gel time of about 10 to15 hours would be required. The gel time is easily controlled bydecreasing the amount of catalyst, from the results it would appear thatdecreasing the catalyst concentration to 0.24% will increase the geltime to 15.5 hours.

DMA indicates that the cured materials (for example Dy(TFMS)₃ catalysedcuring of HTPB) have higher crosslink densities than that obtained forHTPB cured with IPDI (1:1 isocyanate to hydroxyl equivalence). This maybe due to secondary hydroxyls participating in the crosslinking process.

The material cured in the presence of Ce(TFMS)₃ is stiffer than theother five materials. This may be due to the cerium species catalysingthe oxidative crosslinking of the polybutadiene backbone (possibly viathe Ce(IV) salt rather than the Ce(III) salt). The material aged to abrown colour similar to that observed for aged un-stabilised isocyanatecured HTPB.

Group III Metal Trifluoromethanesulfonate Catalysts

HTPB (containing 1% by mass of calco2246 (antioxidant)) was dried invacuo at 50° C. overnight. Yttrium triflate and scandium triflate wereadded to catalyse the reaction between HTPB oligomer and Epikote 828, asper Table 5. The admixture provides a 1:1 mol equivalence of epoxy tohydroxyl groups.

TABLE 5 Curing HTPB with different group III metaltrifluoromethanesulfonate catalysts 1^(st) Gel Order Shear Shear MassTime/ rate Tg/ Modulus at Modulus at Catalyst % hours constant ° C. 60°C./MPa 25° C./MPa Y(TFMS)₃ 0.26 1.11 0.0016 −77 0.53 0.48 Sc(TFMS)₃ 0.480.20 0.010 −77 0.84 0.77 Sc(TFMS)₃ 0.16 5.06 0.0021 −77 0.57 0.51

The above reactions were followed using both infrared spectroscopy(FTIR) and dynamic stress rheology (DSR). The catalytic activity ofscandium triflate was greater than that of the lanthanide triflatecatalysts, as indicated by the gel time and infrared spectroscopy. Theconsequence of this is that lower quantities of scandium triflatecompared to lanthanide triflates are required to catalyse HTPBcrosslinking in an equivalent amount of time.

Energetic Polymer Curing Optimisation

Energetic polymers have been specifically designed to possess a veryhigh heat of combustion compared to traditional polymers (such as HTPB).Therefore, when an energetic composite undergoes reaction, the energeticbinder adds more energy to the output. Three energetic polymers wereinvestigated with regard to the curing procedure: Polyglyn (Glycidylnitrate polymer), GAP (Glycidyl azide polymer) and Polynimmo(3-nitratomethyl-3-methyloxetane polymer).

PolyNIMMO and polyGLYN (2° hydroxy terminated) possess hydroxyfunctionality of less than two hydroxyls per polymer chain. Hence,nominally di-epoxy species such as Epikote 828 will facilitate chainextension rather than crosslinking. Multi functional epoxy crosslinkersare preferred for such materials.

PolyNIMMO

Table 6 below summarises polyNIMMO curing attempts. An excess of epoxywas used for crosslinking. The mixes were cured at 60° C. in a fan ovenfor 7 days.

TABLE 6 PolyNIMMO curing summary. 1st order DMA G′ Catalyst Gel reactionat 60° C./ Catalyst mass % Epoxy oligomer time rate (IR) MPa Sm(TFMS)30.59 Epikote 828 + triphenylol >24 0.0006 0.05 methane triglycidyl ether(1:1 based on epoxy mols) Sm(TFMS)3 0.59 triphenylolmethane 15.1 0.0010.09 triglycidyl ether Yb(TFMS)3 0.53 triphenylolmethane >24 hrs 0.00180.42 triglycidyl ether Sm(TFMS)3 0.59 triphenylolmethane >24 hrs — 0.26triglycidyl ether Dy(TFMS)3 0.60 resorcinol diglycidyl ether >24 hrs0.0017 — Isocyanate — — — — 0.14 cured polyNIMMO

The polynimmo does not possess di-hydroxyl functionality; therefore,admixtures prepared require the use of a higher functionalised epoxyoligomer. PolyNIMMO cured with a trifunctional isocyanate (DesmodurN100) exhibited lower crosslink densities that polyNIMMO cured withtrifunctional epoxy in the presence of lanthanide triflates. That is tosay, the crosslinking was more effective using the epoxy as thecrosslinker rather than the isocyanate.

PolyGLYN

Two forms of PolyGLYN are available—oligomers with secondary hydroxylend groups (e.g. Batch 3.24) or oligomers with secondary and primaryhydroxyl end groups (e.g. Batch BX51).

TABLE 7 PolyGLYN and triphenylolmethane triglycidyl ether, usingEu(TFMS)₃. 1st order DMA G′ DMA G′ Catalyst Gel reaction at 0° C./ at60° C./ Hydroxyl oligomer mass % time rate (IR) MPa MPa PolyGLYN Batch0.63 20.2 0.0010 0.43 0.1 3.24 (1° hydroxyl) PolyGLYN Batch 0.63 >240.00053 1.5 0.14 BX51(2° hydroxyl) PolyGLYN Batch NA NA NA 1.6 0.3BX51-isocyanate cure

Table 7 above, summarises the curing of different batches of polyGLYNwith epoxy crosslinkers. The structure of the endgroups depends on themethod of synthesis. PolyGLYN containing 0.6% by mass Eu(TFMS)₃ catalystwas mixed under vacuum with triphenylolmethane triglycidyl ether in aratio of 1.1 mols epoxy to 1 mol of polyGLYN hydroxyl group. The mixeswere cured at 60° C., in a fan oven, for 7 days.

The mechanical properties of the fully cured materials as shown inTables 1-7 compare well with isocyanate cured samples. The resultsclearly show that hydroxy terminated oligomers crosslinked with epoxyterminated oligomers in the presence of a metal triflate catalyst usedin a method according to the invention, provide crosslinked polymersthat have similar physical properties to the same hydroxyl terminatedoligomers when cured with isocyanates. The method according to theinvention provides a less toxic, more cost effective route to crosslinking hydroxyl terminated oligomers.

The results have further shown that the use of at least one metaltriflate in a method according to the invention provide a means ofsynthesising crosslinked binders which are suitable for use withexplosive materials.

1-25. (canceled)
 26. A method of forming a crosslinked polyether polymercomprising the steps of forming an admixture of at least one hydroxyterminated oligomer, at least one epoxy terminated oligomer and at leastone metal trifluoromethanesulfonate salt catalyst, and curing theresultant admixture at an elevated temperature.
 27. A method accordingto claim 26, wherein at least one of the hydroxy terminated or epoxyterminated oligomers comprises at least 5 to 10% w/w of an oligomerwhich has greater than 2 functional groups.
 28. A method according toclaim 26, wherein at least one of the hydroxy terminated or epoxyterminated oligomers comprises at least 5 to 10% w/w of an oligomerwhich has in the range of 3 to 5 hydroxy or epoxy groups respectively.29. A method according to claim 26, wherein the admixture comprises afurther epoxy terminated oligomer, which contains an average of 2.5 to 4hydroxy groups per oligomer chain, and is present in the range of from5-10% w/w.
 30. A method according to claim 26, wherein the hydroxyterminated oligomer is of formula (i)

wherein A is an oligomer containing monomer repeat units, m is theaverage number of monomer repeat units in the range of from 1 to 10000,x is in the range of from 2 to 20; and the epoxy terminated oligomer isof formula (ii)

wherein B is an oligomer containing monomer repeat units, n is theaverage number of monomer repeat units in the range of from 1 to 10000,and y is in the range of from 2 to
 20. 31. A method according to claim26, wherein the hydroxy terminated oligomer is of formula (i)

wherein x is in the range of from 1 to 3; and the epoxy terminatedoligomer is of formula (ii)

wherein y is in the range of from 1 to 3; and the admixture comprises afurther epoxy terminated oligomer of formula (iii) present in the rangeof from 5-10% w/w,

wherein D is an oligomer containing monomer repeat units, wherein p isthe average number of monomer repeat units in the range of 1 to 10000and z is in the range of from 2.5 to
 3. 32. A method according to claim31, wherein the polyhydroxy terminated oligomer is a dihydroxyterminated oligomer of formula (ia)

wherein the polyepoxy terminated oligomer is di-epoxy oligomer offormula (iia)

and optionally a further epoxy terminated oligomer of formula (iii),


33. A method according to claim 26 wherein the metal is a lanthanide orgroup III metal.
 34. A method according to claim 33, wherein the metalis scandium or yttrium.
 35. A method according to claim 30 wherein A, Band D are independently selected from monomer units comprisinghydrocarbyl, esters, carbonates, ethers, amides, aromatics, heterocyclicor copolymers comprising mixtures thereof.
 36. A method according toclaim 35, wherein the hydrocarbyl is a polydiene.
 37. A method accordingto claim 26 wherein m, n and p are in the range of from 20 to
 50. 38. Amethod according to claim 37, wherein m and p are in the range of from 1to
 10. 39. A method according to claim 26 wherein the curing step iscarried out in the temperature range of from 40 to 85° C.
 40. A methodaccording to claim 39, wherein the temperature is in the range of from40 to 60° C.
 41. A method according to claim 26, wherein the catalyst isadded to the admixture in a minimum quantity of a volatile solvent,wherein said solvent is removed prior to curing the admixture.
 42. Amethod according to claim 26, wherein the admixture is formed in thesubstantial absence of solvent.
 43. A method according to claim 26wherein the catalyst is typically present in an amount of from 0.01% to2% by mass of the reaction mixture.
 44. A method according to claim 26wherein the admixture further comprises at least one filler material.45. A method according to claim 26 wherein the filler is an energeticmaterial.
 46. A method of forming a polymer bonded explosive, comprisingthe steps of forming an admixture as defined in claim 26, uniformlydispersing an energetic material in said admixture and curing theresultant mixture at an elevated temperature.
 47. A kit for use in amethod according to claim 26 comprising the following components eitherall provided separately in the kit, or with any two provided incombination and the third provided separately:— at least one hydroxyterminated oligomer; at least one epoxy terminated oligomer; at leastone metal trifluoromethanesulfonate salt catalyst; and, wherein the kitfurther comprises optional instructions for use.
 48. A kit according toclaim 47 further comprising at least one hydroxy terminated or epoxyterminated oligomer comprising at least 5 to 10% w/w of an oligomerwhich has greater than 2 functional groups.